Matched die mold, using wet molding composites, has been used to produce fiber-reinforced polyurethane foam molded products, such as trunk hoods and autobody parts, seating, helmets, luggage, etc.
This is done by using a matched die having female and male parts which when closed form a mold cavity of the desired contour. With the mold parts opened, a fiber reinforcement, either mat or a preform, is placed on or in the die cavity and a reactive foaming mixture, consisting essentially of polyol and isocyanate, is poured on the reinforcement in a quantity calculated to fill the voids between the reinforcement fibers and the mold cavity completely when the foaming reaction is completed. Closing of the mold compresses the fibrous structure of the reinforcement.
For a molded part of high density and, therefore, strength, it is desirable to use a reinforcement having fine fiber thicknesses and an initial uncompacted thickness proportioned so that when the mold is closed, the reinforcement is very substantially compacted. Normally the initial mixture includes additives such as a catalyst, propellant and possibly other additives.
Such matched die molding of fiber reinforced polyurethane foam molded products has involved the problem that with the dies closed and the reinforcement compacted, the foaming material has been unable to travel or flow through the interstices between the fibers of the reinforcement adequately to insure a good product. To overcome this problem, the prior art has used fiber-reinforcement having coarse fibers which are thicker than is desirable from the strength viewpoint of the ultimate product. The degree of compression of the reinforcement by closing of the die has heretofore necessarily been limited.
The above indicated problem results from the rapid increase in viscosity occurring during the chemical reaction producing the polyurethane foam, preventing flow through the fiber reinforcement unless the latter is designed to have an adequately low flow resistance to the foaming reaction product. With a reinforcing of more desirable fiber thickness and compaction, foam flow distances of only a few centimeters have been experienced, whereas products in the area of more than 2 m in extent form a desirable class of end products. The use of a plurality of sprues or runners to carry the reactive mixture into the mold cavity at an adequately large number of locations loses the advantages of simplicity and low product production costs, of which the matched die molding technique is inherently capable. The use of a molding mixture comprising components which react very slowly with each other, hopefully to effect complete reinforcement penetration with time, prolongs the product manufacturing time so long as to be economically undesirable.
For example, using the polyurethane foam production mixture mentioned earlier, and using a fiber reinforcement which although having undesirably low ultimate product strength characteristics does provide adequate flow of the foaming for products having a size of about 1600 mm, a setting and mold dwell time in the order of four to five minutes is possible. It is undesirable to lengthen this characteristic time period.
The object of the present invention has been to improve on the described molding technique so that relatively large molded products can be produced having a high density and, therefore, strength, and reinforced by fibrous reinforcements comprising fibers of a thinner thickness and compacted to a greater degree to provide the desirable high density, than was previously possible.